This routine is based on the ISO 7064 Mod 97,10 check digit calculation routine.

The two check digit characters in a IBAN number are the third and fourth characters
in the code. For check digit calculation/validation the first four characters are moved
to the end of the code.
So CCDDnnnnnnn becomes nnnnnnnCCDD (where
CC is the country code and DD is the check digit). For
check digit calculation the check digit value should be set to zero (i.e.
CC00nnnnnnn in this example.

Note: the class does not check the format of the IBAN number, only the check digits.